Field of Invention
The invention relates to electrostatic discharge (ESD) protection. Particularly, the invention relates to an ESD protection circuit discharging ESD stress from power rails.
Description of Related Art
In the field of integrated circuit (IC), electrostatic discharge (ESD) protection is an important task on the reliability of the ICs, where a proper ESD protection is needed to discharge any ESD stresses to protect internal circuits from being damaged. Generally, the ESD stresses may occur between pins (pin-to-pin ESD stress), one power rail to another power rail (e.g., VDD-to-VSS ESD stress, VSS-to-VDD ESD stress or domain-to-domain ESD stress).
In the advanced IC technology, ICs are designed to have at least two power domains, for example, one for analog circuits and another for digital circuits, different voltage levels for different digital circuits, etc. In the multiple power domain scheme, extra ESD protection elements are required across the power domains (domain-to-domain protection), as every pin-to-pin ESD combination must be protected.
Conventionally, a silicon controlled rectifier (SCR) or a metal-oxide semiconductor (MOS) transistor is used as ESD clamps to direct and conduct the ESD current properly. However, as both the SCR and the MOS transistor can only operate with a fixed polarity due to the parasitic diodes in their structure, thus they cannot be used for the domain-to-domain ESD protection.
Another commonly used ESD protection element is an antiparallel diode string. The number of diodes on each string may be selected according the voltages on each node in order to avoid conduction under normal circuit operation. However, the antiparallel diodes require large silicon footprint area, especially if the required voltage drop is too large. The diode string is also known to suffer from large leakage current in the nanoscale IC manufacturing processes.
To meet the requirement of conducting the ESD stresses in both directions, another symmetrical device, a dual-SCR (DSCR) device is utilized, which requires smaller silicon footprint than the antiparallel diodes for the same ESD protection level. However, the DSCR device requires the addition of a fully symmetrical triggering circuit. For example, in a ground-to-ground ESD clamp 100 shown in FIG. 1 of the U.S. Pat. No. 7,825,429, trigger elements 114 and 116 integrated by diode strings 110 (1-N) and 112 (1-N) have been utilized to set the trigger voltage of each side of the DSCR device, made of transistors 102, 104, 106 and resistors 108 and 110, at which each diode strings will conduct current and trigger the DSCR device. However, diode strings are known to have a relatively large leakage current in the nanoscale IC manufacturing processes and requires large footprint.